1. Field of the Invention
The present invention relates to a process for producing 2,6-dimethylnaphthalene which is useful as a starting raw material for 2,6-naphthalene dicarboxylic acid. 2,6naphthalene dicarboxylic acid is industrially important as a starting raw material for polyethylene Naphthalate which is employed for producing high-performance polyester in the form of fiber or film having excellent tensile strength and heat resistance.
2. Description of the Related Art
2,6-dimethylnaphthalene (hereinafter dimethylnaphthalene is sometimes abbreviated to "DMN") is employed as a starting raw material for 2,6-naphthalene dicarboxylic acid and is called upon to have a high purity.
DMN has 10 isomers according to the positions of two methyl groups. It is necessary, however, to produce 2,6-DMN substantially free from an isomer other than 2,6-DMN on a large scale at a low cost. It is well known that in the case of isomerizing DMN, isomerization between adjacent .beta.-positions and isomerization of methyl-migration from one ring to another are unlikely to take place as compared with that between .alpha.-position and .beta.-position. Specifically, the abovementioned 10 DMN isomers are classified into four groups, namely A to D groups as undermentioned with regard to isomerization, and isomerization among different groups is unlikely to take place as compared with that in the same group.
Group A -- 1,5-DMN; 1,6-DMN; and 2,6-DMN PA1 Group B -- 1,8-DMN; 1,7-DMN; and 2,7-DMN PA1 Group C -- 1,4-DMN; 1,3-DMN; and 2,3-DMN PA1 Group D -- 1,2-DMN
As the process for producing 2,6-DMN, there are available a process in which naphthalene or methylnaphthalene is methylated, succeedingly isomerized and separated, isolation process from tar fraction or petroleum fraction and the like. However, the fraction or the reaction product according to any of the above-mentioned processes contains almost all the isomers in the aforesaid four groups, thereby necessitating the isomerization among different groups to take place for the purpose of efficiently producing 2,6-DMN by the isomerization of such isomers. As the method of isomerization among different groups, there is disclosed, for example in Japanese Patent Application Laid-Open No. 88433/1984, a method in which is employed a zeolite having a ten-membered oxygen ring at the inlet of the major cavity. However, the above-disclosed method gives rise to a number of side reactions such as disproportionation in addition to the isomerization because of an elevated temperature required for the reaction, thus lowering the yield of useful 2,6-DMN. That is to say, a process for producing 2,6-DMN by isomerizing a mixture of DMN isomers suffers from the disadvantage that a number of DMN isomers contained that are unlikely to be converted to 2,6-DMN because of their belonging to the group other than the group of 2,6-DMN lower the yield of 2,6-DMN, necessitate steps of separation from mixed isomers and thereby make the process inefficient as an industrial process for the production of 2,6-DMN.
On the other hand, Japanese Patent Application Laid-Open Nos. 134634/1974, 89353/1975 and 76852/1973 disclose a process for producing o-tolylpentene-2 in high yield from o-xylene and butadiene; a process for producing 1,5-dimethyltetralin by cyclizing o-tolylpentene-2; and a process for producing 1,5-DMN in high yield and in high selectivity by dehydrogenating 1,5-dimethyltetralin, respectively. In the case where 1,5-DMN thus obtained is used as the starting material for the production of 2,6-DMN, it is advantageous in that 2,6-DMN can be produced without difficult isomerization among different isomer groups, since both 1,5-DMN and 2,6-DMN belong the same group with regard to isomerization.
There are proposed a number of processes for producing 2,6-DMN by isomerizing 1,5-DMN, for example, Japanese Patent Publication No. 50622/1972 discloses a method of isomerization in the gaseous phase by the use of a silica-alumina catalyst, Japanese Patent Publication No. 4008/1983 discloses a method of isomerization in liquid phase by the use of mordenite containing chromium or the like as a catalyst, and U.S. Pat. No. 4962260 discloses a method of isomerization by the use of .beta.-zeolite or Y-type zeolite.
Nevertheless, the method in Japanese Patent Publication No. 50622/1972, although high in terms of the concentration of 2,6-DMN contained in the reaction liquid, has deficiencies in that considerable amounts of 2,7-DMN and 1,7-DMN are produced which are different in isomerization group from 2,6-DMN and further, considerable amounts of monomethylnaphthalene and trimethylnaphthalene are produced by disproportionation. The method in Japanese Patent Publication No. 4008/1983, although low in the production of 2,7-DMN and in side reactions such as disproportionation, suffers the defect that the efficiency of isomerization into 2,6-DMN is limited. In addition, the method in U.S. Pat. No. 4962260, although low in the side reaction such as disproportionation, suffers the disadvantages that the production of isomers belonging to different isomerization groups such as 2,7-DMN group are high and further the efficiency of isomerization into 2,6-DMN is insufficient.
That is to say, it is the present situation that any of the prior art processes can not attain a high efficiency of isomerization into the objective 2,6-DMN, while suppressing the isomerization into a group different from 2,6-DMN group as well as the side reaction such as disproportionation.
The production of 2,7-DMN not only decreases the yield of the objective 2,6-DMN in the isomerization step, but also brings about the loss of 2,6-DMN and lowers the purity thereof in the step of crystallization/separation after the isomerization step because of the formation of the binary eutectic-mixture of 2,7- and 2,6-isomers and the ternary eutectic-mixture of 2,7-, 2,6- and 1,5-isomers. Moreover, the side reaction such as disproportionation leads to a decrease in the yield of 2,6-DMN.
It is well known that various solid acids are useful as the catalyst for the isomerization of an aromatic hydrocarbon. Mordenite is one of solid acids and is frequently used for isomerizing DMN. In addition to the above-mentioned Japanese Patent Publication No. 4008/1983, Japanese Patent Publication No. 11690/1981 discloses a method in which H-type mordenite is employed as the principal catalyst along with bentonite or acid clay as the cocatalyst and Japanese Patent Publication No. 49054/1980 discloses a method in which is used a mordenite having a molar ratio of silica to alumina of 10 to 50.
However, even with the above-disclosed methods, the isomerization into DMN belonging to a different group such as 2,7-DMN group, the side reaction such as disproportionation or the like take place, thus failing to achieve a high efficiency of isomerization into 2,6-DMN.
Under such circumstances, intensive research and investigation were made by the present inventors in order to develop a process for producing 2,6-DMN by isomerizing 1,5-DMN which process is capable of realizing a high efficiency of isomerization from 1,5-DMN to 2,6-DMN, suppressing the formation of DMN belonging to a different group such as 2,7-DMN group as well as the side reaction such as disproportionation, and at the same time, imparting a prolonged service life to the catalyst. As a result, it has been found by the present inventors that it is possible to realize a high efficiency of isomerization into the objective 2,6-DMN, suppress the formation of DMN belonging to a different group such as 2,7-DMN group and the side reaction such as disproportionation and maintain favorable performance over a long period of time by effecting isomerization at a reaction temperature of 270.degree. C. or lower in liquid phase by the use of a catalyst comprising mordenite being substantially of hydrogen form having a molar ratio of silica to alumina (hereinafter referred to as "SiO.sub.2 /Al.sub.2 O.sub.3 ratio") of 100 or higher. The present invention has been accomplished on the basis of the above-mentioned finding.